Biomining can be defined as the use of microorganisms in the recovery of metals from minerals. Its traditional expression is bioleaching, the solubilization of metals from the corresponding sulfide minerals in an acidic medium using the direct or indirect action of microorganisms (Rawlings D. E., MicrobCell Fact. 4(1) (2005) 13).
For example, species from the Acidithiobacillus genre are capable of oxidizing reduced sulfur compounds, such as sulfide, elemental sulfur, thionates, etc. using oxygen as an electron acceptor, which allows the solubilization of metallic ions from minerals. During this process species such as sulfite and thiosulfate are generated as intermediates and sulfuric acid is generated as a final product.
Within the variety of microorganisms that participate in these processes, one of the most studied is Acidithiobacillus ferrooxidans, an acidophilic, autotrophic and quimiolitotrophic bacteria, which means that it lives in environments with acidic pH between 1,3 and 4, it uses CO2 as carbon source and it obtains energy from inorganic compounds. In particular, Acidithiobacillus ferrooxidans oxidizes iron (II) to iron (III) using oxygen as an electron acceptor. Iron (III) is a potent oxidizing agent, that can oxidize reduced sulfur compounds or other reduced compounds (Watling. The bioleaching of sulphide minerals with emphasis on copper sulphides—A review. Hydrometallurgy (2006) vol. 84 (1-2) pp. 81-108).
The usual practice of bioleaching processes in the mining industry consists in charging the mineral previously crushed on an impermeable carpet forming “piles”, which are later irrigated with a diluted solution of sulfuric acid. The solubilized metal in the percolated solution that elutes from the pile (known as Pregnant Leaching Solution or PLS) is then recuperated in successive stages of extraction by solvent and electrodeposition.
Since bioleaching is a microbiological process, its efficiency can be improved by inoculating the mineral with leaching microorganisms. This inoculation can take place when the mineral is charged or during the irrigation of the pile. In the vicinity of bioleaching piles it is possible to install and operate bioreactors for the production of such microorganisms (Morales P., Badilla, R. 2006. “Proceso para aumentar la velocidad de biolixiviación de minerales o concentrados de especies metálicas sulfuradas que comprende inocular continuamente solución de lixiviación que contiene microorganismos aislados de tipo Acidithiobacillus thiooxidans o en conjunto con microorganismos aislados de tipo Acidithiobacillus ferrooxidans”. Solicitud de Patente Chilena N°2006-02911).
It has been described that extracellular polymeric substances (EPS) from Acidithiobacillus ferrooxidans, responsible for bacterial adhesion to the mineral and biofilm formation are crucial for bioleaching. It has been demonstrated that pirite bioleaching by Acidithiobacillus ferrooxidans is significantly greater in bacteria activated with EPS that in those without it (T. Gehrke et al, Appl. Environ. Microbiol. 64 (1998) p. 2743-2747). EPS seems to have 2 roles in bioleaching: (i) mediate bacterial adhesion to the sulfide mineral surface, and (ii) concentrate iron (III) ions in the mineral-microorganism interface by complexation with uronic acids or other EPS residues, allowing the oxidative attack on the sulfur to take place. (Sand W., Gehrke T., Research in Microbiology 157 (2006)49-56).
Having established the importance of EPS in bioleaching, the problem is how to increase EPS production in leaching microorganisms like Acidithiobacillus ferrooxidans. Although the identification of genes involved in EPS formation in Acidithiobacillus ferrooxidans has been accomplished (Barreto et al., Appl. Environ. Microbiol. 71 (2005) 2902-2909), methods to improve its EPS production have not yet been developed.
The present invention tackles the problem of generating microbial cultures that carry metabolic compounds or products that improve the bioleaching rate, proposing a strategy that assures biomass production together with EPS generation or accumulation, hence obtaining a biomass culture with enhanced characteristics, more efficient for bioleaching. The technical problem has been analyzed from a metabolic engineering point of view, developing a mathematical model that represents Acidithiobacillus ferrooxidans' metabolism, based on the genome annotation of Acidithiobacillus ferrooxidans strain Wenelen (Sugio T., Miura A., Parada P., Badilla R. (2005), Cepa bacteriana de Acidithiobacillus ferrooxidans denominada Wenelen, Patent number CL 44546). Simulations developed using the model made possible the determination of which metabolic pathways must be intervene in order to increase EPS specific productivity in Acidithiobacillus ferrooxidans Wenelen.